Various printing methods are used in printing machines. Non-impact printing methods (NIP) are understood as printing methods that do not require a fixed, i.e. physically invariable, printing forme. Printing methods of this type can be used to produce a different print image in every printing operation. Examples of non-impact printing methods include ionographic methods, magnetographic methods, thermographic methods, electrophotography, laser printing and especially inkjet printing methods. Printing methods of this type typically involve at least one image-generating device, for example, at least one print head. In the case of the inkjet printing method, such a print head is embodied as an inkjet print head, for example, and has at least one nozzle, preferably a plurality of nozzles, by means of which at least one printing fluid in the form of ink droplets, for example, can be transferred in a targeted manner to a printing substrate.
The precise matching of a printed image on the front and back sides of a substrate that is printed on both sides is referred to as register (DIN 16500-2). In multi-color printing, the process of combining individual printed images of different colors in a precise correlation to form an image is referred to as color registration (DIN 16500-2). In inkjet printing, as in other printing methods, suitable measures for maintaining color registration and/or register are required. In particular, it is important for the relative position between print head and printing substrate to be known and/or kept constant.
A printing unit in which print heads are mounted on respective supporting bodies is known from WO 2014/184126 A1.
A device by means of which an individual print head can be displaced in a compensation direction is known from US 2013/0127971 A1. If print heads that are mounted adjacent to one another on a common supporting body become displaced relative to one another in the compensation direction as a result of thermal expansion of the supporting body, a lock can be opened, the print head can be displaced relative to the supporting body with respect to the compensation direction by heating and thermal expansion of an expansion block, and the lock can be closed again.